The research objective is to elucidate the molecular basis of the process by which phagocytes ingest particulate objects, the principal mechanism by which these cells defend human hosts against pyogenic infection. The sensory and motor aspects of this basic physiological phenomenon will be studied by means of the following specific projects: 1) The opsonic fragment of the third component of human complement will be purified and analyzed with attention to how it elicits recognition by phagocytic cells; 2) Plasma membranes will be isolated from macrophages, their purity validated, and their interaction with opsonic effector molecules and cytoplasmic contractile proteins investigated; 3) Cytoplasmic contractile proteins of rabbit pulmonary macrophages and human leukemic granulocytes will be purified and their interactions scrutinized, since these proteins provide the mechanical forces for phagocytosis. Particular attention will be focussed on actin, myosin and a recently discovered "actin-binding protein" which promotes the assembly and cross-linking of actin filaments and that may be a "cytoplasmic transducer" for transmitting membrane recognition of ingestible particles into cytoplasmic movement. BIBLIOGRAPHIC REFERENCES: Stossel TP, Hartwig JH: Interactions of actin, myosin and a new actin-binding protein of rabbit pulmonary macrophages II. Role in cytoplasmic movement and phagocytosis. J Cell Biol (March, 1976). Boxer LA, Stossel TP: Isolation and interactions of actin, myosin and an actin-binding protein of myelogenous leukemia granulocytes. J Clin Invest (April, 1976).